Reserved instances OpenSearch

It is applied from the smallest size as described in this document.
So, if you purchase another "m6g.large.search" reserved instance, it may be applied to m6g.xlarge.search.
However, if you are using a smaller size than m6g.xlarge.search, it will be applied in preference to m6g.xlarge.search.
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/apply_ri.html#ri-instance-size-flexibility

With instance size flexibility, the Reserved Instance discount applies to instance usage for instances that have the same family, generation, and attribute. The Reserved Instance is applied from the smallest to the largest instance size within the instance family based on the normalization factor. For an example of how the Reserved Instance discount is applied, see Scenario 2: Reserved Instances in a single account using the normalization factor.

Instance Size Flexibility does not apply to Opensearch instances - https://docs.aws.amazon.com/opensearch-service/latest/developerguide/ri.html

RIs are not flexible; they only apply to the exact instance type that you reserve. For example, a reservation for eight c5.2xlarge.search instances does not apply to sixteen c5.xlarge.search instances or four c5.4xlarge.search instances. For full details, see Amazon OpenSearch Service pricing and FAQ.

If you use 1 m6g.xlarge.search, you will have to buy 1 unit of Opensearch RI for m6g.xlarge.search

In you case - you might request to cancel the RI and request that you do a rebuy of the larger instance type RI for Opensearch.